Method of making composite metal and concrete structures



June 20, 1967 1. H. ROSENBLATT 3,327,028

METHOD OF MAKING COMPOSITE METAL AND CONCRETE STRUCTURES BY 51M wwwATTORNEY June 20, 1967 J. H. RosENBLATT 3,327,028

METHOD OF MAKING COMPOSITE METAL AND CONCRETE STRUCTURES Filed Oct. 19,1964 2 Sheets-Sheet 2 ATTORNEY United States Patent O 3,327,028 METHOD FMAKING CMPSETE METAL AND CONCRETE STRUCTURES .loel H. Rosenblatt, 11619Le Baron Terrace, Silver Spring, Md. 20902 Filed Uct. 19, 1964, Ser. No.404,651 Claims. (Cl. 264-34) This invention relates to improvements incomposite metal and concrete structures suitable for use in theconstruction of bridges, buildings, and the like.

More particularly, the invention is concerned with improvements inconstructions of the above-mentioned type, wherein concrete slabs aresupported by structural metal flexural members and serve as the deck orfloor of the composite structure.

In conventional construction of this type, utilizing a continuousflexural metal structural member, the concrete applied in the region ofnegative bending moment is not considered as participating in thestructural resistance to the stresses of applied loads. The reasons forthis, as will be apparent to those skilled in the art, are essentiallythat the flexural stresses occurring in the region of negative bendingmoment are tensile in direction; and further, that ordinary concretecannot be relied upon to withstand any substantial amount of tension.Accordingly, the strucural value of the concrete in those regions isignored, so far as tensile resistance is concerned, and the actualdesign of the structure, in practice, is predicated upon the assumptionthat the concrete in those regions is cracked.

The principal object of the invention is to provide a method ofconstructing such composite structures, whereby the concrete applied inzones of negative bending moment of a continuous metal structural memberwill participate compositely with the flexural structural member insupporting the loads for which the structure is designed. Thus, by theconstruction of the invention, it becomes possible to utilize, for anygiven load design, flexural metal components of reduced cross-sectionalthickness and hence of lighter weight and lower cost.

In accordance with the invention, stated briefly, the continuousflexural member and the concrete slab are so combined with one anotheras to impart to the composite structure the characteristics of avariable moment Vof inertia beam insofar as concerns the stressdistribution fromsubsequent loading. By thus combining the concrete slabwith the flexural structural member, more of the structure that mayotherwise be structurally Wasted is made useful for supporting theloads.

The invention will be more fully understood from the detaileddescription below and from the accompanying drawing:

FIGS. 1, 2, 3 and 4 are views illustrating several successive stepsutilized in the practise of the invention;

FIG. 5 is a fragmentary View on an enlarged scale, partly incross-section, showing a composite structure of metal and concreteembodying the invention; and

FIG. 6 is a view in perspective, illustrating the structure as itappears in the step illustrated in FIG. 3.

In the drawing, the invention is illustrated as applied to a three-spancontinuous beam, such as might be used in building a bridge.

The beam, indicated by reference numeral 10, may be of any suitableshape, such as the Wide flange I-shape beam shown in FIGS. 5 and 6.

The arrows indicated by numerals 11 and 12, respectively, represent theend supports and the interior supports for the beam.

The top flange of the beam is provided with suitably spaced shearconnectors or transfer elements indicated at 15. These elements 15 maybe of any of the forms of shear connectors commonly employed forestablishing a bonding connection between a metal flexural member and aconcrete slab supported thereby. Thus, these elements 15 may be in theform of angles, channels, headed bolts or pins, or spirals, Welded tothe top surface or the beam for engagement with the concrete of theslab. Desirably, the shear-transfer elements are spaced more closelytogether in the zones of the supports than in mid-span. As will beunderstood, n these elements 15 have suilicient strength and are weldedsecurely enough to mechanically bond the slab to the beam. These shearconnector elements 15 may be welded to the flange of the beam either atthe factory or on the job site, depending upon the particular form ofthe shear connectors used.

FIG. l depicts diagrammatically the beam and the shear connectorsattached thereto in place on its supports indicated by arrows 11, 12.

As depicted in FIG. 2, the slab is poured in sections, the sectionsfirst poured, indicated at 20, extending over the interior supports 12in the regions of normal negative bending moments. As will be understoodby those skilled in the art, during the addition of the dead load of Wetconcrete in the step of the process depicted in FIG. 2, the elasticdistribution of bending moments and dead load stresses thereby inducedis the same as the distribution encountered in conventionalconstruction. The

maximum bending moments for a series of uniformlyl loaded equal spansWill occur as negative bending moments over the interior supports, asindicated by the portions 26 of the dotted line curve representing thedellection pattern of the three-span continuous beam, FIG. 2. Theintervening regions of positive bending moment are indicated by theportions 28 of curved line 27.

In accordance with the invention, pre-stressing tendons 21 comprisingwires or rods of high tensile yield and provided with suitable endfittings 22, are embedded in the concrete of the sections 20.

After the concrete of the slab sections 20 has hardened sufficiently andbefore pouring the concrete for the remaining or intervening sections ofthe slab, the slab sections 20 are pre-stressed by exerting tension inany conventional manner on the pre-stressing tendons 21.

By pre-stressing the slab sections 20 at this stage of the construction,so that they `behave in flexure as elastic material, the concrete isrendered elastically usable at this stage and the structure :therebytransformed, in effect, into that of a continuous Ibea-m with a variablemoment of inertia. The numerous advantageous characteristics of avariable moment of inertia type of beam have long been recognized |byengineers. In order to dbtain these characteristics, it has beennecessary in prior practice to build up the metal member in the vicinityof the interior supports, eil-ther by attaching additional steel platesto the flanges in that area or by fabricating the member so that itbecomes geometrically deeper in this area, or by a combination of both.In contrast thereto, in accordance with Ithe invention, the effect isachieved by utilizing material, i.e., the concrete in the zones ofnegative Ibendingrrnoment, most of which, has heretofore 'been ignoredfor design purposes and its utility thereby wasted.

By thus imparting to the beam 10 the properties of a variable moment ofinertia member, further loading of the structure, as by pouring theremaining sections 24 of the slab, Will result in transferring more ofthe load from the simple lsteel support areas over to the compositesection areas which 'have lbecome proportionally stiffer, due to theincreased moment of inertial of the now composite lsection in the regionof negative bending moment and which consequently Will carry a greatershare of the subsequent loading.

Moreover, by reason of the fact that the sla-b sections 20 in theregions 26 of normal negative bending moments,

are pre-stressed after having been interlocked with the `steel member bymeans of the shear connectors 15 along the interface between the Steeland the concrete, not only is the slab itself pre-stressed, but themetal members 10 themselves are thereby also pre-stressed. Furthermore,the pro-stressing force thereby induced in the metal member not onlywill oppose and thereby reduce the stresses `developed when loads areapplied to the finished structure, but this pre-stressing iniiuence isnot confined to the portions of the metal member in the negative bendingzone in the vicinity of the interior supports, 'but extends throughoutthe length of the metal member in the negative bending zone in thevicinity of the interior supports, but extends throughout the length ofthe metal member. The resultant eect upon the regions of positivebending moment of the beam 10 is `similar to that which may 'be derivedif xed end moments were applied to the positive bending portions of thebeam considered as a free body, and is essentially similar to that of avariable moment of inertia beam, in that some of the mid-span positivebending is transferred out of the mid-span Zone of the beam whereby toreduce the positive ben-ding at mid-span.

As a result of the composite action established between the metal memberand the concrete, the pre-stressing of the concrete in the zones ofnegative bending induces locked-in stresses throughout the steel sectionin directions opposite to the direction of the stresses induced by theloads the structure is intended to sustain. The algebraic sum of thesetwo stresses will be lower than the stresses in the absence of thelpre-stressing force, leaving more capassity for carrying additionalloads within a given limit of total stress.

After the slab sections 2@ have been pre-stressed to the -desireddegree, and the tensioning mechanism has been disconnected from thelittings 22, the concrete for the -remaining slab 4sections 24 is thenpoured to complete the structure.

Instead of pre-stressing the slab sections in the manner describedabove, known as post-tensioning, i.e., wherein the prestressing takesplace after the concrete in these negative bending areas has hardened,it may be desirable in some instances to utilize procedure wherein themetal member 10 is pre-tensioned in these areas before the concrete ofthese slab sections is poured. In accordance with such procedure, an endblock holding device may be attached to the top flange of the beam justoutside of the area of the pour to be pre-stressed and a suitable sizepre-stressing tendon attached thereto and tensioned before the concreteis poured. Thus, at this stage, the naked steel Iof the `beam alone isactually pre-stressed. When the irst pour of concrete is then placed,the de-ad load stresses induced thereby are combined with the-prestressed condition of the steel to establish the stresses at thisstage. After the concrete of these slab sections has hardened, the endholding devices are released, or the tendons may be cut between the edgeof the slab and the end blocks, enabling the stresses to be transferredto the concrete by Way of the shear connectors 15. The end blocks usedas temporary anchorages for the pre-stressing tendons may then beremoved, whereupon completion of the structure may proceed by placing ofthe concrete for the slab sections 24.

Although I have hereinabove referred to the stressing of the slabsections20 (either pre-tensioning or post-tensioning) by exertingtension on wire or :rod tendons 21 embedded in the concrete of sections20, it is to be understood that in lieu of thus stressing thesesections, lsubstantially the same effects as hereinabove set forth maybe obtained by utilizing any known type of concrete composition thatinherently exerts an axial compressive force on the lconcrete of slabsections 20, or that inherently exerts an axial expansive force `fromsections 24 against the concrete of the adjacent slab sections 20.

I claim:

1. A method of combining a concrete slab with a continuous flexuralmember, which comprises iirst applying concrete along transverselyspaced areas constituting sections of the slab in negative bending areasof the continuous exural member to effect a shear-transferring bondbetween said sections of the slab and said flexural member,pre-stressing the said sections of the slab and together therewith thecompositely joined iiexural member, whereby to impart the properties of4a variable moment of inertia beam to the continuous compositestructure, and then applying the concrete to the remaining `sections ofthe slab.

2. A method of constructing a continuous composite concrete andstructural flexural member which comprises placing a metal exural memberon suitably spaced supports, said yilexural member `having sheartransfer devices aixed to the surface thereof which is to constitute theinterface between the concrete and said member, :applying concrete inthe zones of 'said exural member constituting the zones or negativebending moment, imposing stresses in the concrete applied to said zones,whereby to impart the properties of a variable moment of inertia beam tothe composite structure, Iand then applying concrete to the remainingzones of said flexural member.

3. A method of constructing a composite metal `and concrete structure,which `comprises providing a plurality of shear transfer elements alongthe surface of the metal member which is to constitute the interfacebetween the concrete and the metal member, applying concrete in theregions of negative bending moment of said metal member, imposingstresses in the concrete of said regions to induce stresses in the metalmember by shear transfer rfrom the concrete along said interface andapplying concrete to the lremaining zones of `said member.

44. The method Iof claim 1 wherein said pre-stressing of saidfirst-named slab sections atnd the compositely joined flexural member iscarried out by tensioning of tendons incorporated in the concrete ofsaid first-named slab sections.

5. The method of claim 3, wherein 'said imposed stresses of thefirst-named concrete sections and the compositely joined iiexural memberis carried out by tensioning of tendons in said negative bending areasbefore the placing yof the concrete for said first-named sections.

References Cited UNITED STATES PATENTS 2,413,990 l/ 1947 tMuntz 52--2232,510,958 6/1950 Coff 52-223 2,917,901 12/'1959 Lackner 52--223 XRFOREIGN PATENTS 164,961 9/ 1955 Australia.

ROBERT F. WHITE, Prim-ary Examiner.

I. A. FINLAYSON, Assistant Examiner,

1. A METHOD OF COMBINING A CONCRETE SLAB WITH A CONTINUOUS FLEXURALMEMBER, WHICH COMPRISES FIRST APPLYING CONCRETE ALONG TRANSVERSELYSPACED AREAS CONSTITUTING SECTIONS OF THE SLAB IN NEGATIVE BENDING AREASOF THE CONTINUOUS FLEXURAL MEMBER TO EFFECT A SHEAR-TRANSFERRING BONDBETWEEN SAID SECTIONS OF THE SLAB AND SAID FLEXURAL MEMBER,PRE-STRESSING THE SAID SECTIONS OF THE SLAB AND TOGETHER THEREWITH THECOMPOSITELY JOINED FLEXURAL MEMBER, WHEREBY TO IMPART THE PROPERTIES OFA VARIABLE MOMENT OF INERTIA BEAM TO THE CONTINOUS COMPOSITE STRUCTURE,AND THEN APPLYING THE CONCRETE TO THE REMAINING SECTIONS OF THE SLAB.